In short I have had replaced all components in the loop with brand new ones (Room stat, Tank stat, 3 way valve, Programmer, Pump & Brand new Worcester boiler). The boiler was replaced by a Gas Safe Engineer who re-checked all of the wiring and is happy it is wired correctly. After it blew fuses with the new boiler fitted we then tried another 3-way valve and a new programmer and it still blew the fuse.
I have since had an Electrician to look at the cabling in the walls as this seemed the only thing left to check and he Megger tested all the cables and found no cable or insulation faults. He suggested that the fault must be a live to neutral fault as this would blow the fuse and not trip the RCD, whereas a live to earth would trip the RCD.